The present invention relates to a plasma treatment apparatus used for depositing a desired substance on a substrate to form a thin film.
A method of forming a thin film on a predetermined substrate is used in many fields of the art, for example, in the fabrication process of a semiconductor integrated circuit. Generally, a thin film is formed in an atmosphere of high temperature. When fabricating a semiconductor integrated circuit, however, sometimes a thin film is formed on a semiconductor integrated circuit which has been fabricated. (In this case, the semiconductor integrated circuit corresponds to the above-mentioned substrate.) When the thin film is formed on such a semiconductor integrated circuit, a fear that the semiconductor integrated circuit will be negatively influenced if the formation of the thin film is conducted in an atmosphere of high temperature exists. This problem is not limited to the semiconductor integrated circuit but is common to any substrate affected by the high temperatures. In recent years, a microwave plasma treatment apparatus has been proposed as means for solving such a problem. The microwave plasma treatment apparatus will be explained by virtue of FIG. 1A and 1B.
FIG. 1A shows a schematic view of the conventional microwave plasma treatment apparatus and in FIG. 1B shows the distribution of strengths of a magnetic field in the apparatus shown in FIG. 1A. One example of the shown conventional microwave plasma treatment apparatus has been disclosed by U.S. Pat. No. 4,401,054. In FIG. 1A, reference numeral 1 designates a vacuum vessel which is evacuated or exhausted by means of a vacuum pump not shown. Numeral 1a designates a discharge section in the vacuum vessel 1, and numeral 1b a reaction section in the vacuum vessel 1. Numeral 2 designates a waveguide for guiding a microwave emitted from a magnetron not shown, and numeral 3 a microwave introducing window made of an insulating material such as quartz for introducing the guided microwave into the vacuum vessel 1. Numeral 4 designates a supporting plate (or holder) provided at the reaction section 1b, and numeral 5 a substrate which is supported on the supporting plate 4 and on which a thin film is to be formed. Numeral 6 designates a first gas conduit for introducing a first predetermined gas into the discharge section 1a, and numeral 7 a second gas conduit for introducing a second predetermined gas into the reaction section 1b. Numeral 8 designates an electromagnetic coil wound around the vacuum vessel 1 for producing in the vacuum vessel 1 a magnetic field as shown in FIG. 1B.
The operation of the above-mentioned microwave plasma treatment apparatus will now be explained referring to the magnetic field strength distribution shown in FIG. 1B. In FIG. 1B, the abscissa represents a distance in the vacuum vessel 1 from the microwave introducing window 3 and the ordinate represents the strength of the magnetic field produced by the electromagnetic coil 8.
After the vacuum vessel 1 has been evacuated to a pressure not higher than 10.sup.-6 Torr, a predetermined gas, for example, an oxygen gas is introduced at a predetermined pressure from the gas conduit 6 while a microwave is introduced from the microwave introducing window 3, so that a plasma of oxygen gas is produced. At this time, if a magnetic field having a predetermined strength is applied by the electromagnetic coil 8, electrons in the plasma make their circular motions by a Lorentz'force and impinge against non-ionized oxygen molecules which in turn are ionized to release electrons therefrom. In this manner, the ionization of the oxygen gas progresses at a rapid speed through the motions of electrons, thereby generating a plasma of oxygen gas at a high density in the discharge section 1a.
The above-mentioned motions (circular motions) of electrons do not occur under the mere existence of the microwave and the magnetic field but are generated through a resonance phenomenon produced only when a specified magnetic field exists for a specified microwave. Namely, when the introduced microwave has a frequency of 2.45 GHz, the circular motions of electrons take place at a point wherein the strength of the magnetic field is 875 Gauss. Since in FIG. 1B the level of 875 Gauss is represented by broken line and the magnetic field strength distribution in the vacuum vessel 1 is represented by solid line, a point of occurrence of the circular motions of electrons is on a line indicated by one-dotted chain line. Such a point is called an electron cyclotron resonance (ECR) point. A region on the side of the ECR point near the microwave introducing window 3 is called a plasma generation region. The ECR point can also be called an ECR portion since it may have a plane-like or three-dimensional extent.
The plasma thus generated moves toward the reaction section 1b since the evacuation is made. This movement of the plasma is due to the evacuation and since electron ions of positive charges are attracted by a group of electrons of negative charges which have moved through the circular motions thereof. A region on the side of the ECR point near the substrate 5 is called as a plasma transport region. If a predetermined gas, for example, a monosilane gas (SiH.sub.4) is introduced from the gas conduit 7 into the reaction section 1b when the plasma is moving, the monosilane gas impinges against the transported plasma so that the monosilane gas is activated and reacts on the oxygen gas to produce a silicon oxide (SiO.sub.2) which is then deposited on the substrate 5. Thus, a thin film of silicon oxide is formed on the substrate 5.
The thin film formed on the substrate 5 is base on the gases introduced from the conduits 6 and 7.
The above-mentioned microwave plasma treatment apparatus is remarkably excellent where forming a thin film on a substrate liable to be affected by high temperature is desired. However, the efficiency of reaction of a gas as a main raw material introduced from the gas conduit 7 or the rate of deposition of a substance deposited on the substrate 5 and the quality of a thin film formed has not been sufficiently considered.
Another example of the conventional plasma treatment apparatus utilizing the ECR has been disclosed by, for example, JP-A-56-155535 and JP-A-57-79621 and is shown in FIG. 2. In the plasma treatment apparatus shown in FIG. 2, plasma-activated species are produced in a plasma generation chamber 213 and a divergent magnetic field generated by a magnetic field generation coil 204 causes the flow of plasma to impinge upon an object 211 to be treated disposed at a position sufficiently away from a region at which the efficiency of generation of the plasma activated species is the maximum.
In the just-mentioned conventional plasma treatment apparatus, since a vacuum vessel 201 includes the plasma generation chamber 213 and a plasma treatment chamber 214 having a relatively large axial length, as shown in FIG. 2, the size of the vacuum vessel 201 becomes large and hence the size of an exhaust port 206 and the size of the magnetic field generation coil 204 becomes correspondingly large.
Experiments by the present inventors have revealed that in a plasma treatment utilizing the ECR, the treatment characteristics depend on a distance between the ECR position and the object 211 to be treated and improve at smaller distances. The inventors have also found that if the concentration of a gas at the ECR position is made high, a microwave 203 is almost absorbed at the ECR position and does not reach the object 211 so that reflection of the microwave from the object 211, and object supporting base 209, etc. disappears.
When constructing the apparatus shown by FIG. 2, shortening the axial length of the vacuum vessel 201 by positioning the object 211 near a microwave introducing window 210 may be considered. However, if that is done, the reflection of the microwave from the object 211 is present, thereby lowering the efficiency of the treatment with plasma and the plasma treatment characteristics.
The conventional microwave treatment apparatus involving a magnetic field can be classified broadly into two types, i.e. (1) one type in which a magnetic field generating portion is disposed outside of a plasma generation chamber and the flow of plasma produced is applied onto a surface to be treated positioned substantially perpendicular to the direction of a magnetic line of force, as has been disclosed by JP-A-56-155535, and (2) a second type in which a magnetic field generating portion is disposed outside of a vacuum vessel and the flow of plasma is applied onto a surface to be treated positioned substantially parallel to the direction of a magnetic line of force, as has been disclosed by JP-A-58-125820.
In either of the above-mentioned types (1) and (2), since the magnetic field generating portion is disposed outside of the vacuum vessel, it is difficult to more efficiently utilize a magnetic field and to decrease the size of the apparatus. Namely, since a coil or permanent magnet for generating a magnetic field necessary for the generation of a plasma in the vacuum vessel is disposed outside of the vacuum vessel, the magnetic field generated cannot be effectively utilized and hence strengthening the generated magnetic field by supplying a large current through the coil or by making the size of the magnet large is necessary. Therefore, the volume of the treatment apparatus or the floor area occupied by the treatment apparatus becomes undesirably large.
Further, in the above-mentioned types (1) and (2) of apparatuses, the control of the distribution of reactive gases and the distribution deposition seeds in the vacuum vessel are not considered. Therefore, the vacuum vessel is filled with excess reactive gases and deposition seeds so that the film is formed not only on the surface of an object to be treated but also on an inner wall of the vacuum vessel. Therefore, alien or undesired substances (or deposits) are produced in flakes, the maintenance frequency increases, and an undesirably large exhaust system is required for exhausting the excess reactive gases.
In addition, where a cylinder-like structure such as a photosensitive drum is to be treated, the following problems exist.
Namely, in the above-mentioned type (1) disclosed by JP-A-56-155535, the diameter of the vacuum vessel must be made much larger than the dimension of the cylinder-like structure so that a sufficient space of gas ventilation can be established to uniformly distribute deposition of a film to be formed and a flow of the gas and the evacuation can be facilitated. Therefore, the apparatus must be made remarkably large. Further, since only a part of a surface to be treated is treated at a time, total treatment time becomes long.
In the type (2) disclosed by JP-A-58-125820, that the quality of a film formed greatly depends on a distance between the ECR point and a surface to be treated and that the quality of a film formed on a surface greatly distanced from the ECR point becomes unhomogeneous are not considered.